Electrode useable in electrochemical cell and method of making same

ABSTRACT

A method of making an electrode useable in an electrochemical cell, includes the steps of (a) providing an electrically conductive substrate; (b) forming nanostructured current collectors on the conductive substrate; and (c) attaching nanoparticles of a ternary orthosilicate composite to the nanostructured current collectors. The ternary orthosilicate composite includes Li 2 Mn x Fe y Co z SiO 4 , where x+y+z=1.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional application of and claims the benefitof U.S. patent application Ser. No. 13/480,860, filed May 25, 2012,entitled “ELECTRODE USEABLE IN ELECTROCHEMICAL CELL AND METHOD OF MAKINGSAME,” by Weng Poo Kang, Supil Raina, Shao-Hua Hsu and Siyu Wei, whichstatus is allowed, which itself claims the benefit, pursuant to 35U.S.C. §119(e), of U.S. provisional patent application Ser. No.61/491,096, filed May 27, 2011, entitled “LITHIUM-ION BATTERY CATHODECOMPRISING TERNARY COMPOSITE OF NANOSTRUCTURED MATERIALS AND METHODS OFMAKING SAME,” by Weng Poo Kang, Siyu Wei and Supil Raina. Each of theabove applications is incorporated herein by reference in its entirety.

Some references, which may include patents, patent applications andvarious publications, are cited and discussed in the description of thisinvention. The citation and/or discussion of such references is providedmerely to clarify the description of the present invention and is not anadmission that any such reference is “prior art” to the inventiondescribed herein. All references cited and discussed in thisspecification are incorporated herein by reference in their entiretiesand to the same extent as if each reference was individuallyincorporated by reference. In terms of notation, hereinafter, “[n]”represents the nth reference cited in the reference list. For example,[1] represents the 1st reference cited in the reference list, namely, S.Wei, W. P. Kang, J. L. Davidson, B. R. Rogers, and J. H. Huang, ECSTransactions 28, 97 (2010).

STATEMENT AS TO RIGHTS UNDER FEDERALLY-SPONSORED RESEARCH

This invention was made with government support under Contract No.N00014-11-M-0315 awarded by the Office of Navy Research of the UnitedStates. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to a method of making anelectrode useable in an electrochemical cell, and more particularly to amethod of making a battery cathode material including the steps ofproviding an electrically conductive substrate, forming nanostructuredcurrent collectors on the conductive substrate, and attaching (orcoating) nanoparticles of a ternary orthosilicate composite to thenanostructured current collectors.

BACKGROUND OF THE INVENTION

Currently, a lithium-ion battery (LIB) is one of the most promisingbattery technologies that can provide higher energy density than otherbatteries. It also does not suffer from the memory effect and the lossof charge is relatively slow when not in use. Hence, high-performanceLIB remains the preferred technology that would address a much broaderrange of energy source/storage for a variety of applications if advancedcathode material with extreme operating capability could be realized.

Current lithium ion batteries mostly utilize metal oxides as cathodematerial with LiCoO₂ as the most popular and commercially successfulrepresentative [2]. However, due to the intrinsic material properties ofthese metal oxides, further enhancement of LIB performance is limited.Specifically, the metal oxides have limited average potential versusLi/Li⁺, mostly well below 4V except LiMn₂O₄, and most of the metaloxides have the specific capacity well below 180 mAh/g, with theexception of LiNiO₂. The metal oxides are also “hot” cathode materialsdue to the thermal runaway reaction, so there is also a concern forsafety.

Another major group of cathode materials is LiMPO₄, where M=Co, Ni, Mn,or Fe. These materials have the electrode potential in the range ofabout 3.5-5.2 V, but the capacity is still limited below 150-170 mAh/g[3]. Further, these materials have poor electrical conductivity, so theyhave to be made in the form of tiny nanoparticles and coated with acarbon layer, which increases the cost of the materials.

The Li₂MSiO₄ silicate family (where M=Co, Fe, or Mn) has attractedresearch activities for the applications in LIB only recently [4, 5] andmuch work needs to be done to thoroughly understand its properties. Themost significant advantage of this group of materials is the polyanionicstructure with two lithium ions per formula unit. The theoreticalcapacity of these materials is as high as about 330 mAh/g.Unfortunately, Co is an expensive metal despite its high average voltageof about 4.3 V. Therefore, pure Li₂CoSiO₄ is not an efficient andeconomic way for making a cathode. Li₂FeSiO₄ has good cycle-ability, butthe average voltage is only about 3.1 V, far below 4 V. On the contrary,Mn is an inexpensive and abundant element. The average voltage ofLi₂MnSiO₄ is about 4.2 V. The reported specific capacity of Li₂MnSiO₄ isabout 210 and about 250 mAh/g at room temperature and 55° C.,respectively [6]. However, it has to be noted that the entire family ofLi₂MSiO₄ silicates has poor electrical conductivity, therefore Li₂MnSiO₄has to be made into nanoparticles and coated with carbon in order toimprove the conductivity, similar to the aforementioned LiMPO₄. Theadditional carbon-coating process is expensive.

Another major drawback of Li₂MnSiO₄ is its poor cycle life characterizedby the poor capacity retention and rate performance. A recent reportshows a 50% retained capacity at room temperature after 20 cycles. Thepoor cycling performance is mainly attributed to Jahn-Teller distortion,structural instability and low electronic conductivity of the material.Another possible attribution is the electrolyte degradation.

The presence of Mn³⁺ ions in the material system is responsible for thedynamic Jahn-Teller distortion and manganese dissolution, a situationsimilar to that of LiMn₂O₄ spinel cathode. Also, the structure ofLi₂MnSiO₄ is prone to collapsing upon delithiation. During delithiation,a phase separation into MnSiO₄ and Li₂MnSiO₄ may occur, leading to theformation of an amorphous structure, which in turn results in the dropof reversible capacity of the electrode during the cycling.

An effective way to minimize the dynamic Jahn-Teller distortion andprevent the collapse of Li₂MnSiO₄ structure is the utilization of asolid solution of Li₂MnSiO₄ and Li₂FeSiO₄ as the cathode. However, thereis very limited research available on this topic. A few literaturereports do show that addition of Li₂FeSiO₄ has prevented Li₂MnSiO₄ fromcollapsing during delithiation [7, 8]. Nonetheless, according to thereports, the cyclic reversibility is still unacceptable.

Therefore, a heretofore unaddressed need exists in the art to addressthe aforementioned deficiencies and inadequacies.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to an electrode useable inan electrochemical cell. In one embodiment, the electrode has anelectrically conductive substrate, carbon nanotubes (CNTs) in electricalcontact with the conductive substrate, and nanoparticles of aLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite coated on the CNTs, where x+y+z=1.

The conductive substrate includes a thin film formed of an electricallyconductive material, where the thin film is flexible (or rigid). In oneembodiment, the conductive material comprises a metal, an alloy, apolymer, graphite, or a conducting oxide.

In one embodiment, the CNTs have tube diameters in a range of about1.0-1,000.0 nm and height in micrometer range. The nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite have diameters or sizes in a rangeof about 1.0-1000.0 nm.

Additionally, the electrode further has an electrolyte solution filledin spaces among the CNTs and the nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite.

In another aspect, the present invention relates to an electrode useablein an electrochemical cell. In one embodiment, the electrode has anelectrically conductive substrate, nanostructured current collectorsformed on the conductive substrate, and nanoparticles of a ternaryorthosilicate composite coated on the nanostructured current collectors.

The conductive substrate includes a thin film formed of an electricallyconductive material, where the thin film is flexible (or rigid). In oneembodiment, the conductive material comprises a metal, an alloy, apolymer, graphite, or a conducting oxide.

The nanostructured current collectors in one embodiment compriseconductive nanotubes/fibers in electrical contact with the conductivesubstrate. In one embodiment, the conductive nanotubes/fibers includecarbon nanotubes (CNTs) or carbon fibers/nanofibers (CFs). The ternaryorthosilicate composite comprises Li₂Mn_(x)Fe_(y)Co_(z)SiO₄, wherex+y+z=1.

Further, the electrode may also have an electrolyte solution filled inspaces among the nanostructured current collectors and the nanoparticlesof the active material.

In yet another aspect, the present invention relates to anelectrochemical cell comprising the electrode as disclosed above.

In a further aspect, the present invention relates to a method of makingan electrode useable in an electrochemical cell. In one embodiment, themethod includes the steps of providing an electrically conductivesubstrate, forming nanostructured current collectors on the conductivesubstrate, and attaching (or coating) nanoparticles of a ternaryorthosilicate composite to the nanostructured current collectors.

The method may further include the step of filling an electrolytesolution in spaces among the nanostructured current collectors and thenanoparticles of the active material

The conductive substrate comprises a thin film formed of an electricallyconductive material. In one embodiment, the thin film is flexible (orrigid).

In one embodiment, the nanostructured current collectors compriseconductive nanotubes/fibers. The conductive nanotubes/fibers in oneembodiment include carbon nanotubes (CNTs) or carbon fibers/nanofibers(CFs).

In one embodiment, the forming step comprises synthesizing or growingthe conductive nanotubes/fibers, such as CNTs or CFs or other types ofconductive nanotubes/fibers on the conductive substrate.

In one embodiment, the ternary orthosilicate composite comprisesLi₂Mn_(x)Fe_(y)Co_(z)SiO₄, where x+y+z=1.

In one embodiment, the ternary orthosilicate composite is synthesized bya hydrothermal process comprising the steps of mixing startingprecursors of lithium hydroxide, SiO₂ particles, Fe(II) chloridetetrahydrate, manganese chloride, and cobalt chloride, in apredetermined composition ratio to form a mixture, sealing the mixtureunder an Ar (or inert) environment and baking the sealed mixture at apredetermined temperature for a period of time to formLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound, rinsing theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound with de-ionized (DI) water, dryingrinsed Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound in vacuum, ball-milling thedried Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound into nanoparticles, andcalcining the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders in an inert or reducingenvironment.

In another embodiment, the ternary orthosilicate compound is synthesizedby Pechini process comprising the steps of dispersing lithium acetate,SiO₂ particles, citric acid, and ethylene glycol at a firstpredetermined composition ratio in de-ionized (DI) water to form a firstmixture, wherein the mixture is sonicated for a period of time, addingFe(III) citrate, manganese acetate, and cobalt carbonate at a secondpredetermined composition ratio into the first mixture to form a secondmixture, stirring and dwelling the second mixture at a reflux station toform a Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel, drying theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel, grinding the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄into nanoparticles, and calcining the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powdersin an inert or reducing environment.

In another embodiment, the ternary orthosilicate compound is synthesizedby a sol-gel process comprising the steps of dispersing lithium acetate,iron citrate, manganese acetate, and cobalt carbonate at a firstpredetermined composition ratio in de-ionized (DI) water to form a firstmixture, adding citric acid to the first mixture to form a secondmixture, adding tetraethylorthosilicate (TEOS) and ethanol at a secondpredetermined composition ratio to form third mixture, stirring anddwelling the third mixture at a reflux station to form aLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel, drying the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ gelto form a Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound, grinding theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound into nanoparticles, and calcining theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders in an inert or reducing environment.

In one embodiment, the attaching (or coating) step is performed by adripping/wetting process comprising the steps of preparing a suspensionof the nano-particles of the ternary orthosilicate composite in a liquidmedium, dripping the suspension into the nanostructured currentcollectors in electrical contact with the conductive substrate, anddrying the suspension to attach (or coat) the nano-particles of theternary orthosilicate composite onto the nanostructured currentcollectors. The liquid medium comprises acetone, water or other liquidmedia.

These and other aspects of the present invention will become apparentfrom the following description of the preferred embodiment taken inconjunction with the following drawings, although variations andmodifications therein may be affected without departing from the spiritand scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate one or more embodiments of theinvention and together with the written description, serve to explainthe principles of the invention. Wherever possible, the same referencenumbers are used throughout the drawings to refer to the same or likeelements of an embodiment.

FIG. 1A shows schematically a cross-sectional view of a lithium-ionbattery cathode according to one embodiment of the present invention.

FIG. 1B shows schematically a cross-sectional view of thevertical-aligned CNTs according to one embodiment of the presentinvention.

FIG. 1C shows a scanning electron microscope (SEM) diagram ofvertical-aligned CNTs according to one embodiment of the presentinvention.

FIG. 1D shows an SEM image of a side view of the vertical-aligned CNTsaccording to one embodiment of the present invention.

FIG. 1E shows a Raman spectroscopy diagram of vertical-aligned,multi-walled conductive CNTs according to one embodiment of the presentinvention.

FIG. 1F shows a flowchart of a method of making a lithium-ion batterycathode according to one embodiment of the present invention.

FIG. 2 shows schematically a cross-sectional view of a lithium-ionbattery using a cathode according to one embodiment of the presentinvention.

FIG. 3A shows a flowchart of a method of synthesizing the ternaryorthosilicate composite by a hydrothermal process according to oneembodiment of the present invention.

FIG. 3B shows a flowchart of a method of synthesizing the ternaryorthosilicate composite by a Pechini process according to one embodimentof the present invention.

FIG. 3C shows a flowchart of a method of synthesizing the ternaryorthosilicate composite by a sol-gel process according to one embodimentof the present invention.

FIG. 3D shows SEM images of nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite coated CNTs at differentmagnifications according to one embodiment of the present invention.

FIG. 4A shows an SEM image of nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite according to one embodiment of thepresent invention, where x=0.25, y=0.5, and z=0.25.

FIG. 4B shows an SEM image of nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite according to one embodiment of thepresent invention, where x=0.1, y=0.8, and z=0.1.

FIG. 5A shows an energy-dispersive X-ray spectroscopy (EDS) spectrum ofnanoparticles of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite according toone embodiment of the present invention, where x=0.25, y=0.5, andz=0.25.

FIG. 5B shows an EDS spectrum of nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite according to one embodiment of thepresent invention, where x=0.1, y=0.8, and z=0.1.

FIG. 6A shows a DSC plot of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compositeaccording to one embodiment of the present invention, where x=0.25,y=0.5, and z=0.25.

FIG. 6B shows a DSC plot of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compositeaccording to one embodiment of the present invention, where x=0.1,y=0.8, and z=0.1.

FIG. 7 shows XRD spectra of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compositeaccording to one embodiment of the present invention, where x=0.25,y=0.5, and z=0.25 and according to second embodiment of the presentinvention, where x=0.1, y=0.8, and z=0.1.

FIG. 8A shows a Nyquist plot of the impedance of the cathode with theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite at 3V according to one embodiment ofthe present invention, where x=0.1, y=0.8, and z=0.1.

FIG. 8B shows a Nyquist plot of the impedance of the cathode with theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite at 4.7V according to one embodimentof the present invention, where x=0.1, y=0.8, and z=0.1.

FIG. 9A shows a plot of the cell performance of the lithium ion batteryusing the cathode with the Li₂Mn_(0.25)Fe_(0.5)Co_(0.25)SiO₄ compositewith the weight of 4.0 mg according to one embodiment of the presentinvention.

FIG. 9B shows a plot of the cell performance of the lithium ion batteryusing the cathode with the Li₂Mn_(0.25)Fe_(0.5)Co_(0.25)SiO₄ compositewith the weight of 1.6 mg according to one embodiment of the presentinvention.

FIG. 9C shows a plot of the cell performance of the lithium ion batteryusing the cathode with the Li₂Mn_(0.1)Fe_(0.8)Co_(0.1)SiO₄ compositewith the weight of 3.9 mg according to one embodiment of the presentinvention.

FIG. 9D shows a plot of the cell performance of the lithium ion batteryusing the cathode with the Li₂Mn_(0.1)Fe_(0.8)Co_(0.1)SiO₄ compositewith the weight of 1.4 mg and the discharge voltage window of 4.7-2.0 Vaccording to one embodiment of the present invention.

FIG. 9E shows a plot of the cell performance of the lithium ion batteryusing the cathode with the Li₂Mn_(0.1)Fe_(0.8)Co_(0.1)SiO₄ compositewith the weight of 1.4 mg and the discharge voltage window of 4.7-2.4 Vaccording to one embodiment of the present invention.

FIG. 10 shows a plot of the discharge capacity-cycle number relationshipof the lithium ion batteries of FIGS. 9A-9E according to one embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is more particularly described in the followingexamples that are intended as illustrative only since numerousmodifications and variations therein will be apparent to those skilledin the art. Various embodiments of the invention are now described indetail. Referring to the drawings, like numbers indicate like componentsthroughout the views. As used in the description herein and throughoutthe claims that follow, the meaning of “a”, “an”, and “the” includesplural reference unless the context clearly dictates otherwise. Also, asused in the description herein and throughout the claims that follow,the meaning of “in” includes “in” and “on” unless the context clearlydictates otherwise. Moreover, titles or subtitles may be used in thespecification for the convenience of a reader, which shall have noinfluence on the scope of the present invention. Additionally, someterms used in this specification are more specifically defined below.

Definitions

The terms used in this specification generally have their ordinarymeanings in the art, within the context of the invention, and in thespecific context where each term is used. Certain terms that are used todescribe the invention are discussed below, or elsewhere in thespecification, to provide additional guidance to the practitionerregarding the description of the invention. For convenience, certainterms may be highlighted, for example using italics and/or quotationmarks. The use of highlighting has no influence on the scope and meaningof a term; the scope and meaning of a term is the same, in the samecontext, whether or not it is highlighted. It will be appreciated thatsame thing can be said in more than one way. Consequently, alternativelanguage and synonyms may be used for any one or more of the termsdiscussed herein, nor is any special significance to be placed uponwhether or not a term is elaborated or discussed herein. Synonyms forcertain terms are provided. A recital of one or more synonyms does notexclude the use of other synonyms. The use of examples anywhere in thisspecification including examples of any terms discussed herein isillustrative only, and in no way limits the scope and meaning of theinvention or of any exemplified term. Likewise, the invention is notlimited to various embodiments given in this specification.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “directly on” another element, there are nointervening elements present. As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, third etc.may be used herein to describe various elements, components, regions,layers and/or sections, these elements, components, regions, layersand/or sections should not be limited by these terms. These terms areonly used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,a first element, component, region, layer or section discussed belowcould be termed a second element, component, region, layer or sectionwithout departing from the teachings of the present invention.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toanother element as illustrated in the Figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the Figures. Forexample, if the device in one of the figures is turned over, elementsdescribed as being on the “lower” side of other elements would then beoriented on “upper” sides of the other elements. The exemplary term“lower”, can therefore, encompasses both an orientation of “lower” and“upper,” depending of the particular orientation of the figure.Similarly, if the device in one of the figures is turned over, elementsdescribed as “below” or “beneath” other elements would then be oriented“above” the other elements. The exemplary terms “below” or “beneath”can, therefore, encompass both an orientation of above and below.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

As used herein, “around”, “about” or “approximately” shall generallymean within 20 percent, preferably within 10 percent, and morepreferably within 5 percent of a given value or range. Numericalquantities given herein are approximate, meaning that the term “around”,“about” or “approximately” can be inferred if not expressly stated.

As used herein, if any, the term “scanning electron microscope” or itsabbreviation “SEM” refers to a type of electron microscope that imagesthe sample surface by scanning it with a high-energy beam of electronsin a raster scan pattern. The electrons interact with the atoms thatmake up the sample producing signals that contain information about thesample's surface topography, composition and other properties such aselectrical conductivity.

As used herein, if any, the term “energy-dispersive X-ray spectroscopy”or its abbreviation “EDS” refers to an analytical technique used for theelemental analysis or chemical characterization of a sample. It relieson the investigation of an interaction of some source of X-rayexcitation and a sample. Its characterization capabilities are due inlarge part to the fundamental principle that each element has a uniqueatomic structure allowing unique set of peaks on its X-ray spectrum.

As used herein, if any, the term “X-ray diffraction” or its abbreviation“XRD” refers to a method of determining the arrangement of atoms withina crystal or solid, in which a beam of X-rays strikes a crystal anddiffracts into many specific directions. From the angles and intensitiesof these diffracted beams, a crystallographer can produce athree-dimensional picture of the density of electrons within thecrystal. From this electron density, the mean positions of the atoms inthe crystal can be determined, as well as their chemical bonds, theirdisorder and various other information. In an X-ray diffractionmeasurement, a crystal or solid sample is mounted on a goniometer andgradually rotated while being bombarded with X-rays, producing adiffraction pattern of regularly spaced spots known as reflections. Thetwo-dimensional images taken at different rotations are converted into athree-dimensional model of the density of electrons within the crystalusing the mathematical method of Fourier transforms, combined withchemical data known for the sample.

As used herein, if any, the term “differential scanning calorimetry” orits abbreviation “DSC” refers to a thermoanalytical technique in whichthe difference in the amount of heat required to increase thetemperature of a sample and reference is measured as a function oftemperature. Both the sample and reference are maintained at nearly thesame temperature throughout the experiment. Generally, the temperatureprogram for a DSC analysis is designed such that the sample holdertemperature increases linearly as a function of time. The referencesample should have a well-defined heat capacity over the range oftemperatures to be scanned.

As used herein, “nanoscopic-scale”, “nanoscopic”, “nanometer-scale”,“nanoscale”, “nanocomposites”, “nanoparticles”, the “nano-” prefix, andthe like generally refers to elements or articles having widths ordiameters of less than about 1 μm, preferably less than about 100 nm insome cases. In all embodiments, specified widths can be smallest width(i.e. a width as specified where, at that location, the article can havea larger width in a different dimension), or largest width (i.e. where,at that location, the article's width is no wider than as specified, butcan have a length that is greater).

As used herein, a “nanostructure” refers to an object of intermediatesize between molecular and microscopic (micrometer-sized) structures. Indescribing nanostructures, the sizes of the nanostructures refer to thenumber of dimensions on the nanoscale. For example, nanotexturedsurfaces have one dimension on the nanoscale, i.e., only the thicknessof the surface of an object is between 1.0 and 1000.0 nm. Nanotubes havetwo dimensions on the nanoscale, i.e., the diameter of the tube isbetween 1.0 and 1000.0 nm; its length could be much greater. Finally,sphere-like nanoparticles have three dimensions on the nanoscale, i.e.,the particle is between 1.0 and 1000.0 nm in each spatial dimension. Alist of nanostructures includes, but not limited to, nanoparticle,nanocomposite, quantum dot, nanofilm, nanoshell, nanofiber, nanoring,nanorod, nanowire, nanotube, and so on.

As used herein, “plurality” means two or more.

As used herein, the terms “comprising”, “including”, “carrying”,“having”, “containing”, “involving”, and the like are to be understoodto be open-ended, i.e., to mean including but not limited to.

OVERVIEW OF THE INVENTION

The present invention relates to electrodes useable in anelectrochemical cell, methods of making the same, and applications ofthe same. In one embodiment, the electrochemical cell is correspondingto a battery, and the electrode is utilized for a lithium-ion batterycathode and includes a composite of nanostructured materials.

In one aspect of the present invention, an electrode usable for abattery cathode has an electrically conductive substrate, nanostructuredcurrent collectors in electrical contact with the conductive substrate,and nanoparticles of a ternary orthosilicate composite coated on thenanostructured current collectors. The conductive substrate includes athin film formed of an electrically conductive material. Preferably, thethin film is flexible. The conductive material includes a metal, analloy, a polymer, graphite, or a conducting oxide. The nanostructuredcurrent collectors include CNTs or carbon fibers/nanofibers (CFs), wherethe CNTs or CFs are in electrical contact with the conductive substrate.The ternary orthosilicate composite includes Li₂Mn_(x)Fe_(y)Co_(z)SiO₄,where x+y+z=1. The CNTs have diameters or thicknesses in a range ofabout 1.0-1,000.0 nm. The nanoparticles of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄composite have diameters or sizes in a range of about 1.0-1000.0 nm.

In addition, the electrode further includes an electrolyte solutionfilled in spaces among the CNTs and the nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite.

In one embodiment, CNTs are utilized as an array of nano-architecturecurrent collectors formed directly on a conductive substrate, which canbe a flexible (or rigid) conducting foil (e.g. metal, graphite).Vertical-aligned CNTs impregnated with MnO₂ nano-particles as electrodes[1] have been used for electrochemical supercapacitors recently, whichresulted in excellent performance of about 1,000 F/cm³. In the presentinvention, the material coated on the CNT array is a high-performanceactive layer of ternary orthosilicate compound with composition ofLi₂Mn_(x)Fe_(y)Co_(z)SiO₄, where x+y+z=1. The electrode using suchnanostructured CNTs provides a high surface area of attachment forLi₂Mn_(x)Fe_(y)Co_(z)SiO₄nanoparticles, which minimizes the contactresistance at the active material/current collector interface, andthereby maximizes the charge efficiency and the energy density of thecathode.

The electrode of the present invention is a high-voltage, high-capacity,and inexpensive cathode for lithium-ion batteries (LIBs) capable ofsupporting high transient and pulsed loads while offering enhancedsafety and lifecycle performance. Currently LIB is one of the mostpromising battery technologies that can provide higher energy densitythan other battery technologies. It also does not suffer from the memoryeffect and the loss of charge is relatively slow when not in use. Hence,with the electrode of the present invention, high-performance LIB can berealized to address a much broader range of energy source/storage forboth military and civil applications.

Li₂MSiO₄, where M=Mn, Fe, and/or Co, is similar to olivine phosphate(LiFePO₄) and has low electron conductivity. Fabrication processes ofthe LIB cathode structures utilizing such materials usually involvemaking the active material in the form of tiny nanoparticles, mixingthem with carbon or coating them with a conductive carbon layer, andthen pasting the mixture onto a conductive substrate (e.g., an aluminumfoil) with a binder material. However, the electron conductivity of themixture is limited by the high resistivity of the binder material andthe high ohmic contact resistance between the cathode material and thesubstrate. In addition, the heat transfer between the cathode materialand the substrate is also less than optimal, leading to elevated cathodetemperature under heavy load.

Accordingly, the present invention utilizes a ternary orthosilicatecomposite, such as the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite, for thecathode active material, which would lead to a cathode with high voltage(≧4V), high capacity (≧180 mAh/g), excellent cycle life, and low cost.To achieve the present invention, synthesis of high performance ternaryorthosilicate composite of Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ must be realizedsuch that the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite can be attached to theCNTs structure. Thus, the cathode obtained would have enhanced specificarea and minimized interface resistance, thereby maximizing the chargetransfer efficiency and specific power-energy densities.

In a further aspect of the present invention, a method of making acathode useable in a battery includes the steps of providing anelectrically conductive substrate, forming nanostructured currentcollectors on the conductive substrate, and attaching nanoparticles of aternary orthosilicate composite to the nanostructured currentcollectors.

In one embodiment, the ternary orthosilicate composite is synthesized bya hydrothermal process comprising the steps of mixing startingprecursors of lithium hydroxide, SiO₂ particles, Fe(II) chloridetetrahydrate, manganese chloride, and cobalt chloride, in apredetermined composition ratio to form a mixture, sealing the mixtureunder an Ar environment and baking the sealed mixture at a predeterminedtemperature for a period of time to form Li₂Mn_(x)Fe_(y)Co_(z)SiO₄compound, rinsing the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound with de-ionized(DI) water, drying rinsed Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound in vacuum,ball-milling the dried Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders intonanoparticles, and calcining the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders in aninert or reducing environment.

In another embodiment, the ternary orthosilicate composite issynthesized by a Pechini process comprising the steps of dispersinglithium acetate, SiO₂ particles, citric acid, and ethylene glycol at afirst predetermined composition ratio in de-ionized (DI) water to form afirst mixture, wherein the mixture is sonicated for a period of time,adding Fe(III) citrate, manganese acetate, and cobalt carbonate at asecond predetermined composition ratio into the first mixture to form asecond mixture, stirring and dwelling the second mixture to form aLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel, drying the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel,grinding the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound into nanoparticles, andcalcining the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders in an inert or reducingenvironment.

In another embodiment, the ternary orthosilicate compound is synthesizedby a sol-gel process comprising the steps of dispersing lithium acetate,iron citrate, manganese acetate, and cobalt carbonate at a firstpredetermined composition ratio in de-ionized (DI) water to form a firstmixture, adding citric acid to the first mixture to form a secondmixture, adding tetraethylorthosilicate (TEOS) and ethanol at a secondpredetermined composition ratio to form third mixture, stirring anddwelling the third mixture at a reflux station to form aLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel, drying the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel,grinding the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ into nanoparticles, and calciningthe Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders in an inert or reducingenvironment.

The attaching step, in one embodiment, is performed by adripping/wetting process comprising the steps of preparing a suspensionof the nano-particles of the ternary orthosilicate composite in a liquidmedium, dripping the suspension into the nanostructured currentcollectors in electrical contact with the conductive substrate, anddrying the suspension to attach the nano-particles of the ternaryorthosilicate composite onto the nanostructured current collectors. Theliquid medium comprises acetone, water or other liquid media.

The present invention in one aspect also relates to a battery comprisingthe cathode as disclosed above.

These and other aspects of the present invention are more specificallydescribed below.

IMPLEMENTATIONS AND EXAMPLES OF THE INVENTION

Without intent to limit the scope of the invention, exemplary methodsand their related results according to the embodiments of the presentinvention are given below. Note that titles or subtitles may be used inthe examples for convenience of a reader, which in no way should limitthe scope of the invention. Moreover, certain theories are proposed anddisclosed herein; however, in no way they, whether they are right orwrong, should limit the scope of the invention so long as the inventionis practiced according to the invention without regard for anyparticular theory or scheme of action. It should be appreciated thatwhile these techniques are exemplary of preferred embodiments for thepractice of the invention, those of skill in the art, in light of thepresent disclosure, will recognize that numerous modifications can bemade without departing from the spirit and intended scope of theinvention.

Example One The Cathode Structure and Method of Making the Same

FIG. 1A shows schematically a cross-sectional view of a lithium-ionbattery cathode according to one embodiment of the present invention. InFIG. 1A, the cathode 100 is a novel CNT-based cathode structure, whichincludes an electrically conductive substrate 110, single walled ormulti-walled CNTs 120 in electrical contact with the conductivesubstrate 110, and nanoparticles 130 of an Li₂Mn_(x)Fe_(y)Co_(z)SiO₄composite attached to the CNTs 120.

In one embodiment, the cathode 100 further includes an electrolytesolution 140 filled in spaces among the CNTs 120 and the nanoparticles130 of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite.

In one embodiment, the conductive substrate 110 can be a flexible (orrigid) thin film formed of an electrically conductive material, such asa metal, an alloy, a polymer, graphite, or a conducting oxide. Forexample, the conductive substrate 110 can be a flexible (or rigid) thinfilm aluminum or graphite foil.

The CNTs 120 serve as nanostructured current collectors of the cathode.In one embodiment, the CNTs 120 are grown directly on the conductivesubstrate 110. The direct growth of the CNTs 120 on the conductivesubstrate 110 provides strong chemical bonding at the interface betweenthe CNTs 120 and the conductive substrate 110 such that the contactresistance is minimized. In one embodiment, the CNTs 120 have diametersor thicknesses in a range of about 1.0-1,000.0 nm. For example, FIG. 1Bshows schematically a cross-sectional view of the CNTs according to oneembodiment of the present invention. As shown in FIG. 1B, the CNTs 120have diameters of about 30 nm, and the distance between adjacent CNTs120 is about 40 nm. FIGS. 1C and 1D show scanning electron microscope(SEM) images of vertical-aligned CNTs according to one embodiment of thepresent invention.

The CNTs, particularly multi-walled CNTs, are highly conductive. FIG. 1Eshows a Raman spectroscopy diagram of vertical-aligned, multi-walledconductive CNTs according to one embodiment of the present invention. Asshown in FIG. 1E, the strongest peak at 1327 cm⁻¹ corresponds to theD-band in the CNT structures, and the second strongest band at 1587 cm⁻¹corresponds to G-band graphite mode. The other bands located at 2654 and2915 cm⁻¹ are due to the second-order combinations of 2D and D+G,respectively.

The Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite is a ternary orthosilicatecomposite in the group of Li₂MSiO₄, where M=Mn, Fe, or Co. In oneembodiment, x+y+z=1. In one embodiment, the nanoparticles 130 of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite have diameters or sizes in a rangeof about 1.0-1,000.0 nm.

As stated above, due to the extremely high surface area of the CNTs 120(or the nanostructured current collectors), the total surface area ofthe current collectors of the cathode 100 becomes three-dimensionalinstead of two-dimensional and increases thousands of times. As aresult, both electric charge and heat transfer of the cathode 100 becomemuch more efficient, and capacity and safety against overheating of thecathode 100 are thus improved.

In some embodiments, carbon fibers/nanofibers (CFs) may replace the CNTsas the nanostructured current collectors. The structure and performanceof the CFs are similar to those of the CNTs, and detailed description ofthe CFs is hereafter omitted.

In some embodiments, other type of nanotubes/fibers (such as conductivemetal-oxide nanotubes/fiber) may replace the CNTs as the nanostructuredcurrent collectors, with the structure and performance similar to thoseof the CNTs.

FIG. 1F shows a flowchart of a method of making a lithium-ion batterycathode according to one embodiment of the present invention. Accordingto FIG. 1F, an electrically conductive substrate, such as the conductivesubstrate 110 in FIG. 1, is provided (step S110), and nanostructuredcurrent collectors, such as the CNTs 120 in FIG. 1A or the CFs, areformed or vertically in electrical contact with the conductive substrate(step S120). Then, nanoparticles of a ternary orthosilicate composite,such as the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite, are attached to thenanostructured current collectors (step S130) to form the cathode. Insome embodiments, a further step can be added to fill an electrolytesolution in spaces among the nanostructured current collectors and thenanoparticles of the active material.

The forming or growing of the CNTs can be performed, for example, by amicrowave plasma-enhanced chemical vapor deposition (CVD) process. Toassist the forming of the CNTs, a thin layer of Ni or Co or a suitablecatalyst can be deposited on the conductive substrate as catalyst. Inone embodiment, hydrogen-diluted methane or a suitable hydrocarbon canbe used as the carbon source.

Example Two Lithium-Ion Battery Comprising the Cathode

FIG. 2 shows schematically a cross-sectional view of a lithium-ionbattery using a cathode according to one embodiment of the presentinvention. In FIG. 2, the battery 200 includes a cathode 202, an anode204, and a separator 206 between the cathode 202 and the anode 204. Thecathode 202 is similar to the cathode 100 in FIG. 1A, which includes anelectrically conductive substrate 210, CNTs 220 in electrical contactwith the conductive substrate 210, nanoparticles 230 of anLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite attached to the CNTs 220, and anelectrolyte solution 240 filled in spaces among the CNTs 220 and thenanoparticles 230 of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite. The anode204 includes an electrically conductive substrate 250, anode structures260 in electrical contact with the conductive substrate 250, and anelectrolyte solution 270 filled in spaces among the anode structures260.

In one embodiment, the conductive substrate 210 of the cathode 202 canbe a flexible (or rigid) thin film formed of an electrically conductivematerial, such as a metal, an alloy, a polymer, graphite, or aconducting oxide. For example, the conductive substrate 210 can be aflexible (or rigid) thin film aluminum or graphite foil.

The CNTs 220 serve as nanostructured current collectors of the cathode.In one embodiment, the CNTs 220 are single walled or multi-walledconductive CNTs formed directly on the conductive substrate 210. In oneembodiment, the CNTs 220 have diameters or thicknesses in a range ofabout 1.0-1,000.0 nm.

The Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite is a ternary orthosilicatecomposite. In one embodiment, x+y+z=1. In one embodiment, thenanoparticles 230 of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compositenanoparticles have diameters or sizes in a range of about 1.0-1,000.0nm.

The anode 204 can be any type of anode. In one embodiment, theconductive substrate 250 of the anode 204 can be a flexible (or rigid)thin film formed of an electrically conductive material, such as ametal, an alloy, a polymer, graphite, or a conducting oxide. Forexample, the conductive substrate 250 can be a flexible (or rigid) thinfilm copper foil.

The anode structures 260 serve as nanostructured current collectors ofthe anode. In one embodiment, the anode structures can be formed of highcapacity anode materials, such as silicon nanowires.

In one embodiment, the electrolyte solutions 240 and 270 can be the sameelectrolyte solution, such as lithium bis(trifluoromethanesulfonyl)imide(LiTFSI) electrolyte.

Example Three

Synthesis of Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ Solid Solution

An advantage of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite as the ternaryorthosilicate composite used in the cathode exists in that both theaverage potential difference of the cathode and the specific capacity ofthe cathode can be increased. As indicated above, conventional metaloxides used as cathode materials have limited average potential versusLi/Li⁺, mostly well below 4V, and most of the metal oxides have thespecific capacity well below 180 mAh/g. In other words, these metaloxide cathode materials cannot meet both the requirements of the averagepotential difference being larger than 4V and the specific capacitybeing larger than 180 mAh/g.

In contrast, the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite not only allows thespecific capacity to reach about 330 mAH/g and provides adequate averagepotential difference of at least 4V, but also has a satisfactory cyclelife with original capacity. Thus, using the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄composite as the cathode material may lead to better performance andlonger life cycle of the battery.

There are three major synthesis methods of the ternary orthosilicatecomposite, such as the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite. One is ahydrothermal synthesis process, the second is the Pechini synthesisprocess, and the third is the sol-gel process.

FIG. 3A shows a flowchart of a method of synthesizing the ternaryorthosilicate composite by a hydrothermal process according to oneembodiment of the present invention. As shown in FIG. 3A, startingprecursors of lithium hydroxide, SiO₂ particles, Fe(II) chloridetetrahydrate, manganese chloride, and cobalt chloride are mixed togetherin a predetermined composition ratio to form a mixture (step S310). Thenthe mixture is sealed under an Ar environment, and the sealed mixture isbaked at a predetermined temperature for a period of time to formLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound (step S312). TheLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound is then rinsed with de-ionized (DI)water (step S314), and the rinsed Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound isdried in vacuum (step S316). The dried Li₂Mn_(x)Fe_(y)Co_(z)SiO₄compound is ball-milled into nanoparticles (step S318) and finally, theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders are calcined in an inert or reducingenvironment (step S319).

FIG. 3B shows a flowchart of a method of synthesizing the ternaryorthosilicate composite by a Pechini process according to one embodimentof the present invention. As shown in FIG. 3B, lithium acetate, SiO₂particles, citric acid, and ethylene glycol are dispersed at a firstpredetermined composition ratio in de-ionized (DI) water to form a firstmixture, and the first mixture is sonicated for a period of time (stepS320). Then Fe(III) citrate, manganese acetate, and cobalt acetate at asecond predetermined composition ratio are added into the first mixtureto form a second mixture (step S322). The second mixture is then stirredand dwelled to form a Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel (step S324), and theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ sol-gel is dried to formLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound (step S326). The driedLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound are grounded into nanoparticles (stepS328). Finally, the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders are calcined in aninert or reducing environment (step S330).

FIG. 3C shows a flowchart of a method of synthesizing the ternaryorthosilicate composite by a sol-gel-process according to one embodimentof the present invention. As shown in FIG. 3C, at step S340, lithiumacetate, iron citrate, manganese acetate and cobalt carbonate aredispersed at a first predetermined composition ratio in de-ionized (DI)water to form a first mixture. At step S341, citric acid is added to thefirst mixture to form a second mixture. Then tetraethylorthosilicate(TEOS) and ethanol are added at a second predetermined composition ratioto form a third mixture (at step S342). At step S343, the third mixtureat a reflux station is stirred and dwelled to form aLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel. At step S344, theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel is dried to form aLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound. At step $345, theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound is grinded into nanoparticles. Then,the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders are calcined in an inert orreducing environment.

An example of synthesis of Li₂FeSiO₄ is used to describe the threesynthesis processes as follows.

For the hydrothermal process, the starting precursors of lithiumhydroxide, SiO₂ particles, and Fe(II) chloride tetrahydrate are mixed inthe predetermined composition ratio of 4:1:1 to form the mixture. Themixture is sealed under the Ar environment and baked at 150° C. for 14days to form the Li₂FeSiO₄ powders. The Li₂FeSiO₄ powders are thenrinsed with de-ionized (DI) water, dried in vacuum, and ball-milled intonanoparticles.

For the Pechini process, lithium acetate, SiO₂ particles, citric acid,and ethylene glycol are dispersed at the first predetermined compositionratio of 2:1:2:1 in the DI water to form the first mixture, and thefirst mixture is sonicated for 2 hours. Then Fe(III) citrate is added tothe first mixture (at the second predetermined composition ratio of1:0:0 since neither Mn nor Co is used) to form the second mixture. Afterfurther stirring and dwelling of the second mixture, the Li₂FeSiO₄ gelis formed, which is then dried and ground into nanoparticles. Finally,the Li₂FeSiO₄ powders are heat treated in the CO/CO₂ environment togenerate the end product.

For the sol-gel process, lithium acetate and iron citrate are dispersedat the pre-determined ratio of 2:1 in de-ionized (DI) water to form afirst mixture. Saturated solution of citric acid is added to the firstmixture to form a second mixture. Tetraethylorthosilicate (TEOS) andethanol are added at a second predetermined composition ratio to formthird mixture which is stirred at 80° C. for 14 hours at a refluxstation to form Li₂FeSiO₄ gel. The Li₂FeSiO₄ gel is dried and groundinto nanoparticles. Finally, the Li₂FeSiO₄ powder is calcined in anArgon atmosphere.

The three processes can be applied for the synthesis of the ternaryorthosilicate composite, such as Li₂Mn_(x)Fe_(y)Co_(z)SiO₄, where Mn, Feand Co all exist in the composite. In one embodiment, the ternaryorthosilicate composite obtained can be subsequently dispersed onto theCNTs by a dripping or wetting method. Specifically, a suspension of thenanoparticles of the ternary orthosilicate composite can be prepared ina liquid medium. Then, by dripping the suspension into thenanostructured current collectors in electrical contact with theconductive substrate and drying the suspension, the nanoparticles of theternary orthosilicate composite are coated onto the nanostructuredcurrent collectors. By varying the number of droplets applied in thedripping process, the mass ratio of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄composite to the CNTs can be controlled.

In one embodiment, the liquid medium includes acetone, water or otherliquid media.

FIG. 3D shows SEM images of nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite attached to the CNTs at differentmagnifications according to one embodiment of the present invention.

The performance of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite can beverified with material characterization analysis, such as SEM,energy-dispersive X-ray spectroscopy (EDS), differential scanningcalorimetry (DSC) and X-ray diffraction (XRD) analysis.

FIGS. 4A and 4B show two SEM images of nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite according to one embodiment of thepresent invention. The two figures represent two embodiments. In FIG.4A, x=0.25, y=0.5, and z=0.25, referring to the ternary orthosilicatecomposite of Li₂Mn_(0.25)Fe_(0.5)Co_(0.25)SiO₄. In FIG. 4B, x=0.1,y=0.8, and z=0.1, referring to the ternary orthosilicate composite ofLi₂Mn_(0.1)Fe_(0.8)Co_(0.1)SiO₄. FIGS. 5A and 5B shows energy-dispersiveX-ray spectroscopy (EDS) diagrams of the nanoparticles of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite in FIGS. 4A and 4B, respectively.FIGS. 6A and 6B shows differential scanning calorimetry (DSC) diagramsof the nanoparticles of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite in FIGS.4A and 4B, respectively.

An expected composite ratio in the form of Mn:Fe:Co of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite in FIGS. 4A and 5A is 1:2:1(0.25:0.5:0.25). However, as shown in FIG. 5A, quantitative analysis ofthe EDS data shows that the composite ratio is about 1.6:1:1.5. Anexpected composite ratio in the form of Mn:Fe:Co of theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite in FIGS. 4B and 5B is 1:8:1(0.1:0.8:0.1). However, as shown in FIG. 5A, quantitative analysis ofthe EDS data shows that the composite ratio is about 1:10:1. Both FIGS.6A and 6B show that, under calorimetric measurements, theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composites have good thermal behavior withnegligible exothermic release at the temperature under 500° C.

FIG. 7 shows XRD spectra of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compositesaccording to two embodiments of the present invention, where x=0.25,y=0.50, z=0.25 and, where x=0.1, y=0.8, z=0.1.

FIGS. 8A and 8B show diagrams of the impedance of the cathode with theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite at 3V and 4.7V, respectively,according to one embodiment of the present invention, where x=0.1,y=0.8, and z=0.1. Excess coating of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄composite on the CNTs would lead to high equivalent series resistance(ESR) and degrading discharge capacity.

Example Four Performance of the LIB

In order to show the performance of the LIB of the present invention,five embodiments of the LIB using cathodes with different compositionratio and weight of the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ composite anddifferent discharge voltage window are provided.

FIG. 9A shows a diagram of the cell performance of the lithium ionbattery using the cathode with the Li₂Mn_(0.25)Fe_(0.5)Co_(0.25)SiO₄composite with the weight of 4.0 mg according to one embodiment of thepresent invention. In this embodiment, the discharge voltage window is4.7-2.0 V. For the performance obtained, the discharge capacity is 63-80mAh/g, the specific energy is 180-263 Wh/kg, and the specific power is178 W/kg. As shown in FIG. 9A, the cathode performance of thisembodiment is unsatisfactory due to the composition ratio and too thickamount by weight of the ternary orthosilicate composite coating on theCNTs.

FIG. 9B shows a diagram of the cell performance of the lithium ionbattery using the cathode with the Li₂Mn_(0.25)Fe_(0.5)Co_(0.25)SiO₄composite with the weight of 1.6 mg according to one embodiment of thepresent invention. In this embodiment, the discharge voltage window is4.7-2.0 V. For the performance obtained, the discharge capacity is100-109 mAh/g, the specific energy is 337-369 Wh/kg, and the specificpower is 439 W/kg. As shown in FIG. 9B, the cathode performance of thisembodiment is improved comparing to the embodiment of FIG. 9A due tothinner amount by weight of the ternary orthosilicate composite coatingon the CNTs.

FIG. 9C shows a diagram of the cell performance of the lithium ionbattery using the cathode with the Li₂Mn_(0.1)Fe_(0.8)Co_(0.1)SiO₄composite with the weight of 3.9 mg according to one embodiment of thepresent invention. In this embodiment, the discharge voltage window is4.7-2.0 V. For the performance obtained, the discharge capacity is150-163 mAh/g, the specific energy is 467-518 Wh/kg, and the specificpower is 178 W/kg. As shown in FIG. 9C, the cathode performance of thisembodiment is improved comparing to the embodiment of FIG. 9A due toadjustment of the composite ratio of the ternary orthosilicate compositecoating on the CNTs.

FIG. 9D shows a diagram of the cell performance of the lithium ionbattery using the cathode with the Li₂Mn_(0.1)Fe_(0.8)Co_(0.1)SiO₄composite with the weight of 1.4 mg and the discharge voltage window of4.7-2.0 V according to one embodiment of the present invention. In thisembodiment, the discharge voltage window is 4.7-2.0 V. For theperformance obtained, the discharge capacity is 288-325 mAh/g, thespecific energy is 919-984 Wh/kg, and the specific power is 489 W/kg. Asshown in FIG. 9D, the cathode performance of this embodiment is greatlyimproved comparing to the previous embodiments of FIGS. 9A-9C due toadjustment of the composite ratio and thinner amount by weight of theternary orthosilicate composite coating on the CNTs.

FIG. 9E shows a diagram of the cell performance of the lithium ionbattery using the cathode with the Li₂Mn_(0.1)Fe_(0.8)Co_(0.1)SiO₄composite with the weight of 1.4 mg and the discharge voltage window of4.7-2.4 V according to one embodiment of the present invention. In thisembodiment, the discharge voltage window is 4.7-2.4 V. For theperformance obtained, the discharge capacity is 182-209 mAh/g, thespecific energy is 695-764 Wh/kg, and the specific power is 485 W/kg.The average voltage of the battery is 4.0-4.05 V. As shown in FIG. 9E,the cathode performance of this embodiment is also greatly improvedcomparing to the previous embodiments of FIGS. 9A-9C. The high dischargecapacity is larger than 180 mAh/g and the specific energy is larger than600 Wh/kg at the average voltage of larger than 4V. Further, the batteryis stable in that the discharge capacity maintains more than 90% of itsoriginal discharge capacity of 209 mAh/g at the voltage range of 4.7-2.4V within 10 cycles. The discharge capacity is also close to itstheoretical limit of 330 mAh/g for 4 cycles in the voltage range of4.7-2.1 V.

FIG. 10 shows a diagram of the discharge capacity-cycle numberrelationship of the lithium ion batteries of FIGS. 9A-9E according toone embodiment of the present invention. In FIG. 10, the lines 1010,1020, 1030, 1040 and 1050 respectively represent the lithium ionbatteries of FIGS. 9A-9E. As shown by the lines 1040 and 1050 in FIG.10, high performance lithium ion batteries, such as the batteries inFIGS. 9D and 9E, can achieve extremely high discharge capacity of largerthan 180 mAh/g within 10 cycles at voltage range of 4.7-2.4 V, which islarger than 90% of its original first discharge capacity of 209 mAh/g.Further, the battery of FIG. 9E can achieve deep discharge capacity ofabout 330 mAh/g in the voltage range of 4.7-2.1V, which is close to itstheoretical limit of 330 mAh/g.

The foregoing description of the exemplary embodiments of the inventionhas been presented only for the purposes of illustration and descriptionand is not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Many modifications and variations are possiblein light of the above teaching.

The embodiments were chosen and described in order to explain theprinciples of the invention and their practical application so as toenable others skilled in the art to utilize the invention and variousembodiments and with various modifications as are suited to theparticular use contemplated. Alternative embodiments will becomeapparent to those skilled in the art to which the present inventionpertains without departing from its spirit and scope. Accordingly, thescope of the present invention is defined by the appended claims ratherthan the foregoing description and the exemplary embodiments describedtherein.

REFERENCE LIST

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What is claimed is:
 1. A method of making an electrode useable in anelectrochemical cell, comprising the steps of: (a) providing anelectrically conductive substrate; (b) forming nanostructured currentcollectors on the conductive substrate; and (c) attaching nanoparticlesof a ternary orthosilicate composite to the nanostructured currentcollectors.
 2. The method of claim 1, wherein the conductive substratecomprises a film formed of an electrically conductive material.
 3. Themethod of claim 2, wherein the film is flexible or rigid.
 4. The methodof claim 1, wherein the nanostructured current collectors compriseconductive nanotubes/fibers.
 5. The method of claim 4, wherein theconductive nanotubes/fibers comprie carbon nanotubes (CNTs) or carbonfibers (CFs).
 6. The method of claim 4, wherein the forming stepcomprises growing the conductive nanotubes/fibers on the conductivesubstrate.
 7. The method of claim 1, wherein the ternary orthosilicatecomposite comprises Li₂Mn_(x)Fe_(y)Co_(z)SiO₄.
 8. The method of claim 7,wherein x+y+z=1.
 9. The method of claim 7, wherein the ternaryorthosilicate composite is synthesized by a hydrothermal processcomprising: (a) mixing starting precursors of lithium hydroxide, SiO₂particles, Fe(II) chloride tetrahydrate, manganese chloride, and cobaltchloride, in a predetermined composition ratio to form a mixture; (b)sealing the mixture under an Ar environment and baking the sealedmixture at a predetermined temperature for a period of time to formLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound; (c) rinsing theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound with de-ionized (DI) water; (d)drying rinsed Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound in vacuum; (e)ball-milling the dried Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound intonanoparticles; and (f) calcining the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powder inan inert or reducing environment.
 10. The method of claim 7, wherein theternary orthosilicate composite is synthesized by a Pechini processcomprising: (a) dispersing lithium acetate dehydrate, SiO₂ particles,citric acid, and ethylene glycol at a first predetermined compositionratio in de-ionized (DI) water to form a first mixture, wherein themixture is sonicated for a period of time; (b) adding Fe(III) citrate,manganese acetate, and cobalt carbonate at a second predeterminedcomposition ratio into the first mixture to form a second mixture; (c)stirring and dwelling the second mixture to form aLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel; (d) drying the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄gel to form Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound; (e) grinding theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders into nanoparticles; and (f)heat-treating the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders in an inert orreducing atmosphere.
 11. The method of claim 7, wherein the ternaryorthosilicate compound is synthesized by a sol-gel process comprisingthe steps of: (a) dispersing lithium acetate, iron citrate, manganeseacetate and cobalt carbonate at a first predetermined composition ratioin de-ionized (DI) water to form a first mixture; (b) adding citric acidto the first mixture to form a second mixture, (c) addingtetraethylorthosilicate (TEOS) and ethanol at a second predeterminedcomposition ratio to form a third mixture; (d) stirring and dwelling thethird mixture at a reflux station to form a Li₂Mn_(x)Fe_(y)Co_(z)SiO₄gel; (e) drying the Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ gel to form aLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound; (f) grinding theLi₂Mn_(x)Fe_(y)Co_(z)SiO₄ compound into nanoparticles; and (g) calciningthe Li₂Mn_(x)Fe_(y)Co_(z)SiO₄ powders in an inert or reducingenvironment.
 12. The method of claim 1, wherein the attaching step isperformed by a dripping/wetting process comprising: (a) preparing asuspension of the nano-particles of the ternary orthosilicate compositein a liquid medium; (b) dripping the suspension into the nanostructuredcurrent collectors in electrical contact with the conductive substrate;and (c) drying the suspension to attach the nano-particles of theternary orthosilicate composite onto the nanostructured currentcollectors.
 13. The method of claim 12, wherein the liquid mediumcomprises acetone, water or other liquid media.
 14. The method of claim1, further comprising filling an electrolyte solution in spaces definedamong the nanostructured current collectors and the nanoparticles of theactive material.